cmake: set correct path to gitconfig on Windows

[dennisameling]

Closes #3071

Currently, when Git for Windows is built with CMake, the gitconfig system-level file can't be read by Git. Because of this, things like git clone don't work correctly. More details in #3071 (comment). This PR fixes that behavior by mimicking what was already done in config.mak.uname, but then slightly changed to work correctly with CMake.

Note: this applies to all CMake builds where Windows is targeted (WIN32).

Before applying this PR:

denni@DESKTOP-8HTP3NV ARM64 /
$ git config --list --system
fatal: unable to read config file 'C:/Program Files (x86)/Git/arm64/etc/gitconfig': No such file or directory

After applying this PR:

denni@DESKTOP-8HTP3NV ARM64 /
$ git config --list --system
pack.packsizelimit=2g
diff.astextplain.textconv=astextplain
filter.lfs.clean=git-lfs clean -- %f
filter.lfs.smudge=git-lfs smudge -- %f
filter.lfs.process=git-lfs filter-process
filter.lfs.required=true
http.sslbackend=openssl
http.sslcainfo=C:/Program Files (x86)/Git/mingw32/ssl/certs/ca-bundle.crt
core.autocrlf=true
core.fscache=true
core.symlinks=true
pull.rebase=false
credential.helper=manager-core
credential.https://dev.azure.com.usehttppath=true
init.defaultbranch=main

[dscho]

That looks pretty good. Only the commit message seems to be eager to be improved ;-)

This commit moves the gitconfig on Windows to /etc/gitconfig, which is where Git expects it. For more details, see #3071 (comment)

How about this instead?

cmake(windows): set correct path to the system Git config

Currently, when Git for Windows is built with CMake, the system Git config is
expected in a different location than when building via `make`: the former
expects it to be in `<runtime-prefix>/mingw64/etc/gitconfig`, the latter in
`<runtime-prefix>/etc/gitconfig`.

Because of this, things like `git clone` do not work correctly (because cURL is
no longer able to find its certificate bundle that it needs to validate HTTPS
certificates). See the full bug report and discussion here:
https://github.com/git-for-windows/git/issues/3071#issuecomment-789261386.

This commit aligns the CMake-based build by mimicking what is already done in
`config.mak.uname`.

This closes https://github.com/git-for-windows/git/issues/3071.

[dennisameling]

Sounds good to me! Just updated the commit message 😊